1. Field of the Invention
The present invention pertains to an offshore well structure including a support caisson and wellhead assembly and a method of installing the caisson and wellhead assembly.
2. Background
In the installation of offshore oil and gas wells in relatively shallow waters conventional practice includes, initially, installing a so-called drivepipe which is driven to refusal. The well is then drilled and, if it is successful, it is completed. After the well completion a supporting caisson is over-driven around the drivepipe before the drilling rig is moved off of the drill site. The caisson and the drivepipe or the wellhead assembly are suitably inter-connected and the caisson serves as a support structure which is sufficiently strong to withstand wind and wave action against the free standing drivepipe and casing arrangement. U.S. Pat. No. 4,710,061 to C. R. Blair et al and assigned to the assignee of the present invention discloses typical prior ar offshore well structures.
Prior art practice has also required several steps in installing and removing a device known as a diverter with respect to the drivepipe during each stage of drilling to permit the installation of casing strings of progressively smaller diameter. The prior art practice also, of course, requires the final step of overdriving the protective caisson after the well is completed. Considering the high cost of drilling and completing oil and gas wells, including offshore wells in particular, there has been a continuing desire to reduce the number of components required for the well installation as well as the time required to drill and complete the well. The present invention is directed to an improved offshore wellhead system of the so-called free standing type as well as an improved method for installing same.